| Boiling Point No. 01 - Special Edition 1989 |
Summary of reports by Ian McChesney et al of bnquetting research project funded by the ODA and implemented by ITDG in partnership with BEST-UK, AFRC-UK' ENERTEC-KENYA, commenced 1986.
To field test in Kenya a small-scale, Kenyan sawdust briquetting plant designed specifically to produce domestic fuel for the urban markets of developing countries.
PHASE I consisted of the basic materials research needed for the partial pyrolysis of sawdust, an appraisal of the wider implications of the partial pyrolysis process and the market studies needed for the Kenyan field studies.
PHASE II (Jan 1988-Sept 1989)
Stage I saw the installation, development and testing of partial pyrolysis and briquetting equipment in UK using UK and Kenyan sawdust.
After the addition of insulation, the unit was considered to be satisfactory with a thermal efficiency of 50% and a heat energy consumption of 1,200 kJ/kg.
The reports compared the technical and economic merits of hot and cold feeds to the extrusion press and to the piston press. It was concluded that with a hot feed screw press, briquetting could meet all the criteria specified. A sawdust feeder to the pyrolyser and an outlet control were found necessary. Smoke generated during briquette formation caused problems. Operator experience was needed for satisfactory operation and consistent briquettes. Briquette quality depends on two main properties - degree of pyrolysis and degree of compaction. These properties are ultimately determined by the type of raw materials used. Moisture content must be controlled.
The criteria specified for the briquetting system are:
- mechanical power consumption with hot feed to the press unit to be not more than 40% of cold feed or 4() kWh/tonne;
- mild steel die with 40 hours between maintenance and a life of 500 hours.
Attempts to modify the standard, all metal, charcoal Jiko to burn batches of solid briquettes were not successful and work was concentrated on a special briquette-burner the "Rocket Stove" described in BP16, illustration reproduced below. This uses hollow, "cylindrical" briquettes and will not burn wood or charcoal satisfactorily so that a reliable supply of the briquettes must be ensured for users. The stove is cheap, simple and easy to make and use and met the following criteria "stove to accept charges of 0.5 to 1.0 kg. of briquettes, give controllable power levels of 2.4 kW, smoke free within 10 minutes of ignition and attendance levels no more than charcoal stoves". Only with the "Briquette Burner" were the ignition, smoking and attendance criteria met. Its performance was found to be comparable in terms of output, controllability and ease of use with Kenyan Jikos. However, better performance, less smoke and higher power levels are desirable.
Cooking cost criteria required costs to be less than with charcoal. The briquette burner was comparable.
Current work (to be completed by Sept. 1989)
Work is continuing on the following:
- Production of three tons of briquettes from Kenyan sawdust by the AFRC, UK research unit.
- Kenyan "market" trials with these briquettes using Rocket Stoves and Jikos at commercial briquette prices.
The outstanding problems are smoke pollution, pyrolyser hot spots and safety, starting up and output. Input material (e.g. particle size and moisture content) and operating methods are still critical with the present experimental set up.
In preparation for the market trials, several prototype stoves were constructed and tested on briquette samples in Nairobi. While overall burning performance of the 'rocket' layout remains good, the short tests confirmed that at the high altitude the initial power level of the stove becomes marginal. To address this shortcoming, stove design has been returned to the original layout of three briquette tubes and further testing will be undertaken.
The recognised difficulties of marketing a new fuel/stove combination have been minimised by the original idea of adopting the rocket configuration as a modification to the widely sold charcoal Jiko. Essentially, the top of the stove remains a standard charcoal unit while the base incorporates the modifications necessary to burn briquettes. This effectively means that the new unit can be sold into the replacement stove market with a minimal premium for the briquette conversion.
Market Trial - While the prime justification of briquetting projects is their ability to contribute positively to the fuel economy - greater supply, lower prices, etc. - this is an insufficient basis on which to market the briquettes. Unless individual households positively decide to adopt briquettes then there will be no market. Households base their cooking decisions on a number of criteria, and of these actual stove efficiency may have a very low priority.
While stove testing always focuses on efficiency, the objective of the marketing study is to determine whether or not there will be effective demand for the briquettes. From previous observations, consumers place particular importance on the speed of a stove in raising food to cooking temperature. This implies relatively high power levels, often in exchange for poorer emission characteristics. Altitude factors in Nairobi tend to reduce the power level of a stove and the rocket configuration has been successfully modified to provide, if necessary, these operating characteristics.
Ed. note: The final conclusions of the project will be reported in a future edition of Boiling Point.